The present invention relates to constant-temperature combustion and a compound cycle and engine for operating on same.
Current combustion processes typically involved in constant-volume, constant-pressure or limited-pressure cycles occupy a small portion of the piston expansion stroke. Actual fuel combustion constitutes an extremely small portion of the cycle, thus generates a high firing temperature, but is not sufficient for complete combustion. As a result, current combustion processes promote formation of NOx and other harmful greenhouse gas.
Some internal combustion engines employ recycled exhaust gas (EGR engines) to lower the firing temperatures thereof and reduce NOx formation. However, the exhaust from EGR engines can not meet Federal emission standards without treating same with, for example, a catalytic converter.
Catalytic converters are labyrinthine duct-like structures lined with or constructed from materials that absorb undesired elements from exhaust coursing therethrough. Catalytic converters may be damaged or rendered ineffective when exposed to sulfur. To protect catalytic converters from sulfur damage, fuel combusted in the associated internal combustion engine must be treated to remove sulfur. De-sulfurizing fuel is expensive and problematic.
Accordingly, reducing NOx production without the expense or other difficulties occasioned by independent fuel or exhaust treatments, ideally, should address the combustion phase of an internal combustion engine cycle.
The combustion process may be described in terms of the ideal gas law:
PV=(M/n)*RTxe2x80x83xe2x80x83(1)
where P is the pressure of the gas, V is the volume thereof, M is the mass thereof, n is the molecular weight thereof and T is the temperature thereof. When the volume of a perfect gas changes from a first volume V1 to a second volume V2, the ratios of the final pressure to the initial pressure, and the final temperature to the initial temperature are derived from:
P2/P1=(V1/V2)kxe2x80x83xe2x80x83(2)
T2/T1=(V1/V2)(kxe2x88x921)xe2x80x83xe2x80x83(3)
where k is equal to CP/CV, CP being the specific heat at constant pressure and CV being the specific heat at constant volume. These ratios demonstrate that temperature changes much slower than the pressure with respect to the same volume change. It follows that, for the same volumetric expansion, far less heat is required to maintain a constant temperature than to maintain a constant pressure constant. Thus, for the same amount of heat added, a much larger volumetric expansion is needed to maintain constant temperature than to maintain constant pressure.
Also, maintaining constant temperature during combustion prolongs the time during which fuel actually is combusted, thus achieving more complete fuel combustion, which improves overall combustion efficiency.
Further, when the firing pressure is equal to or less than the compression pressure, the fuel-air mixture in the combustion chamber will have less tendency to leak into or remain in crevices and escape combustion. Equal firing and compression pressure also suppresses the tendency of the temperature behind the flame front from increasing due to increased pressure, which would promote NOx formation.
What is needed, and not taught or suggested by the prior art, is a method and an engine for promoting constant-temperature combustion.
The invention overcomes the limitations discussed above and provides a method and an engine for promoting constant-temperature combustion.
The invention provides for prolonging the time during which fuel actually is combusted during a combustion process, thereby improving overall combustion efficiency
The invention limits firing pressure to be equal to or less than the compression pressure, thereby reducing major pollutant formation mechanisms.
To this end, the invention is a method for combusting fuel in an engine involving decreasing a first volume of a gas to a second volume while increasing a pressure and a temperature thereof, then increasing the second volume to a third volume at constant pressure while adding heat until a predetermined temperature is obtained, and finally increasing the third volume to a fourth volume while decreasing the pressure at the predetermined temperature. Increasing the third volume is accompanied by adding more heat, in an amount that sustains constant-temperature combustion. The invention also is a compound engine including a limited-temperature cycle internal combustion engine which produces exhaust and a Lenoir cycle apparatus operated by the exhaust.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments which refers to the accompanying drawings.